Effect of Repeated Vaccination With the Same Vaccine Component Against 2009 Pandemic Influenza A(H1N1) Virus

• Published in: The Journal of infectious diseases 2017-03, Vol.215 (6), p.847-855
• Main Authors: Martínez-Baz, Iván, Casado, Itziar, Navascués, Ana, Díaz-González, Jorge, Aguinaga, Aitziber, Barrado, Laura, Delfrade, Josu, Ezpeleta, Carmen, Castilla, Jesús
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Aged; Aged, 80 and over; Case-Control Studies; Child; Child, Preschool; Female; Humans; Influenza A Virus, H1N1 Subtype; Influenza Vaccines - administration & dosage; Influenza Vaccines - therapeutic use; Influenza, Human - epidemiology; Influenza, Human - prevention & control; Logistic Models; Male; Middle Aged; Spain; Vaccination; VIRUSES; Young Adult
• ISSN: 0022-1899; 1537-6613
• DOI: 10.1093/infdis/jix055

Background. The 2009 pandemic influenza A(H1N1) (A[H1N1]pdm09) vaccine component has remained unchanged from 2009. We estimate the effectiveness of current and prior inactivated influenza (AH1N1)pdm09 vaccination from influenza seasons 2010–2011 to 2015–2016. Methods. Patients attended with influenza-like illness were tested for influenza. Four periods with continued A(H1N1)pdm09 circulation were included in a test-negative design. Results. We enrolled 1278 cases and 2343 controls. As compared to individuals never vaccinated against influenza A(H1N1) pdm09, the highest effectiveness (66%; 95% confidence interval, 49%–78%) was observed in those vaccinated in the current season who had received 1–2 prior doses. The effectiveness was not statistically lower in individuals vaccinated in the current season only (52%) or in those without current vaccination and >2 prior doses (47%). However, the protection was lower in individuals vaccinated in the current season after >2 prior doses (38%; P = .009) or those currently unvaccinated with 1–2 prior doses (10%; P < .001). Current-season vaccination improved the effect in individuals with 1–2 prior doses and did not modify significantly the risk of influenza in individuals with >2 prior doses. Conclusion. Current vaccination or several prior doses were needed for high protection. Despite the decreasing effect of repeated vaccination, current-season vaccination was not inferior to no current-season vaccination.